Most military vehicles that are employed with tactical units and provide and receive communications by high frequency (HF) radio, typically utilize tapered flexible vertical antennas called "whips". The most common whips consist of four 4 foot sections (16 foot whip) for use while the vehicle is moving, or eight 4 foot sections (32 foot whip) for use when the vehicle is stationary, referred to as "at-halt" operation. The sections are typically joined with threaded fittings. The bottom section has a threaded fitting for attachment to an antenna mount attached to a vehicle or shelter. The vertical orientation whip is practical for vehicle mounting and useful for short distance ground wave communications. However, certain intermediate distance communication requires that NVIS (near vertical incidence skywave) communication be employed. This involves refraction of the radio signals off the ionosphere at angles near 90.degree. above the horizontal. Signals emitted at high vertical radiation angles are reflected/refracted from the ionosphere at acute angles and return to earth at short and medium distances with useable signal intensity. NVIS is particularly effective where the participating net stations are spread over geographical areas within approximately 300 miles of each other. For example, if HF radio stations operating on lower HF frequencies (2 to 14 MHz) radiate signals at angles between 90 degrees (directly overhead) to approximately 45 degrees, the signals will return to earth with a signal 300 miles (480 kilometers) of the transmitting station.
To produce adequate signal levels at these high angles, sending and receiving antennas should be horizontally polarized. The whip antenna, in its normal position, is vertically polarized, i.e., the electrostatic field is perpendicular to the Earth and the electromagnetic field is parallel to the Earth, thus producing low signal levels at high angles. The vertical radiation pattern of a vertical whip has the highest gain at angles below 45 degrees above the horizon. The antenna thus performs fairly well when used for ground wave communication (short distances, usually under 25 miles), but poorly at high radiation angles necessary for NVIS communication.
One method of providing some horizontal polarization for NVIS operation is to bend the whip from the vertical toward a horizontal position to the maximum extent possible. However, because the bottom whip sections are rigid and spring mounts (when used) are stiff, it is difficult to bend the lower sections of the whip to a near horizontal position where maximum current and radiation occurs.
Adjustable antennas for vehicles are known in the art. U.S. Pat. Nos. 4,109,251, 4,243,989, 4,827,273, 4,101,897, 4,055,845, and 4,074,271 each disclose an adjustable antenna mounted on a vehicle. The antennas used in the mountings disclosed in these patents are not whip antennas. Further, these types of adjustable antennas are bulky, expensive, and difficult to use. It would be impractical to use the types of antennas disclosed in the above-mentioned patents in place of the standard whip antenna.
U.S. Pat. Nos. 2,934,764, 2,979,729 and 4,625,213 each disclose mounts for antennas. The mounts hold an antenna to a surface in a fixed orientation, and do not provide for easy transition between vertical and horizontal polarization. Further, the disclosed mounts cannot readily be substituted for a mount on an existing whip antenna connection.
There is therefore a need to provide a method and simple and inexpensive device for changing the polarization of a whip antenna between vertical and horizontal without requiring replacement of the vehicle's existing whip or radio antenna coupler.